CRYSTAL GROWTH & DESIGN
Morphology and Structure of Chain-Folded Lamellar Crystals of Nylons 2 22, 4 22, 6 22, 8 22, 10 22, and 12 22
2004 VOL. 4, NO. 2 383-387
Guosheng Zhang and Deyue Yan* College of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China Received September 23, 2003;
Revised Manuscript Received December 18, 2003
ABSTRACT: Chain-folded lamellar crystals of nylons 2 22, 4 22, 6 22, 8 22, 10 22, and 12 22 are grown from 1,4-butanediol solution, and their morphology and crystalline structure are investigated by using transmission electron microscopy (TEM) and wide-angle X-ray diffraction (WAXD). It is found that nylons 4 22, 6 22, 8 22, 10 22, and 12 22 have a similar crystalline structure. All of them crystallize into the triclinic structure of the nylon 6 6 Rp phase, and their unit cell parameters are also similar to those of nylon 6 6. On the other hand, nylon 2 22 is special in the crystalline structure compared with other ones. Although nylon 2 22 also adopts the triclinic Rp phase, its characteristic signals of diffraction are different from other nylons of this series and nylon 6 6, but similar to those of the nylon 2 Y family. Furthermore, its parameters of the unit cell calculated from the experimental data come to the same conclusion. Introduction Nylons, linear aliphatic polyamides, are one kind of semicrystalline polymer, which can form different crystalline phases. There are strong hydrogen bonds between amide groups in adjacent molecular chains of nylons, which results in the formation of chain-folded and hydrogen-bonded sheets.1 For the even-even nylons, due to the requirement that all hydrogen bonds must be linear, the molecular chains within these sheets can progressively shear by 13° parallel to the chain axis (p mode).2-5 On the other hand, the hydrogen-bonded sheets can stack together,2-8 either by progressive shear known as R phase or with alternating shear termed the β structure by Bunn and Garner9 who studied nylon 6 6 and nylon 6 10. In addition, the pseudohexagonal phase is another crystalline form, which is generally observed in some nylons at high temperature.10 Both R and β crystalline structures give two strong and characteristic signals in wide-angle X-ray diffraction (WAXD) patterns at spacings of 0.44 and 0.37 nm, respectively. Furthermore, the two signals indexed as 100 and 010/110 reflections represent a projected interchain distance within hydrogen-bonded sheets and intersheet spacing, respectively.9 However, there is only one strong characteristic signal at spacing of 0.42 nm in the WAXD patterns of the pseudohexagonal system.10 It is well-known that both the physical properties and structure of nylons are dependent on the number of methylene groups in the monomeric units. Compared with some traditional nylons, the nylons with long alkane segments have some special properties, so there is increasing interest in them.2,4,8,11-14 In this paper, the chain-folded lamellar crystals of a series of nylons with long diacid alkane segments were prepared by dilute solution crystallization, and their morphology and crystalline structure were investigated by transmission electron microscopy (TEM) and WAXD techniques. * To whom correspondence should be addressed. Tel: +86-2154742665. Fax: +86-21-54741297. E-mail:
[email protected] Table 1. Conditions for Preparing the Single Crystals of Nylons 2 22, 4 22, 6 22, 8 22, 10 22, and 12 22 nylons
T1 (°C)
T2 (°C)
Tm (°C)
nylon 2 22 nylon 4 22 nylon 6 22 nylon 8 22 nylon 10 22 nylon 12 22
190 190 200 200 210 210
155 155 150 150 145 145
216 207 189 180 173 170
Experimental Section Materials and Preparation. Nylons 2 22, 4 22, 6 22, 8 22, 10 22, and 12 22 were synthesized by melt polycondensation of organic salts of 1,20-cosanedicarboxylic acid and the corresponding diamine. The materials have the molecular weights between 5 000 and 15 000 as determined by gel permeation chromatogram (GPC). The single crystals of the nylons were prepared from a solution of 1,4-butanediol (0.05% w/v) by seeding at T1 and crystallizing at T2 as listed in Table 1. The sedimented mats for WAXD measurements were recovered by centrifugation and washed repeatedly with n-butanol, while the films of the nylons were stretched at 90 °C for imaging plate photographs of WAXD. On the other hand, the melting temperatures of the sedimented mats (Tm) determined by Perkin-Elmer Pyris-1 differential scanning calorimeter (DSC) are also listed in Table 1 as a comparison. Instrumentation. Lamellar crystals of the nylons were examined by using a HITACHI H-800 TEM (175 kV) in both imaging and electron diffraction (ED) modes. A drop of the crystal suspension was deposited onto carbon-coated copper grids, and the solvent was removed by evaporation in a vacuum at 80 °C for 24 h; then the samples can be used for TEM measurements. WAXD measurements were performed using a Rigaku III Dmax 2500V PC X-ray diffractometer with Cu radiation (40 kV, 250 mA). Silicon was used for calibration purposes, and imaging plate photographs were taken with the camera diameter of 75 mm.
Results and Discussion Due to the special structure of lamellar crystals for nylon 2 22 compared with other prepared nylons, it will be discussed separately. First, morphology and crystal-
10.1021/cg034177r CCC: $27.50 © 2004 American Chemical Society Published on Web 01/28/2004
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Table 2. Spacings of ED and WAXD for Nylons 4 22, 6 22, 8 22, 10 22, 12 22, and 2 22 electron diffraction (nm)
X-ray diffraction (nm)
nylons
d100
d010/110
d100
d010/110
d001
d002
nylon 4 22 nylon 6 22 nylon 8 22 nylon 10 22 nylon 12 22
0.440 0.443 0.445 0.442 0.442
0.373 0.372 0.373 0.371 0.373
0.443 0.443 0.443 0.446 0.444
0.369 0.370 0.371 0.371 0.372
2.561 ( 0.001 2.737 ( 0.001 2.972 ( 0.005 3.158 ( 0.005
1.278 ( 0.005
nylon 2 22
0.413
0.390
0.414
0.390
2.740 ( 0.001
line structure of nylons 4 22, 6 22, 8 22, 10 22, and 12 22 are discussed as below. Morphology and Crystalline Structure of Nylons 4 22, 6 22, 8 22, 10 22, and 12 22. Figure 1 shows the typical morphologies of the lamellar crystals observed with the TEM imaging mode, of which nylons 4 22 and 8 22 are given as examples. All of the lamellar crystals prepared in our conditions are elongated and multilayered, similar to those reported for many other eveneven nylons.2-8 In addition, the long sides of the lamellar crystals show serrated edges. However, the morphology of nylon 4 22 (Figure 1a) is lath-shaped, and it is more regular than other four nylons in this series which are a sheaf with lath-shaped extremities similar
Figure 1. Examples of transmission electron micrographs of the lamellar crystals: (a) nylon 4 22, (b) nylon 8 22.
1.578 ( 0.005 1.696 ( 0.005 1.368 ( 0.001
to nylon 8 22 (Figure 1b). On the other hand, the size of the lamellar crystals for nylons 4 22, 6 22, 8 22, 10 22, and 12 22 is about several microns in length and several hundred nanometers in width. The lamellar crystals of these nylons bring almost the same selected area ED patterns, one of which is shown in Figure 2. There is only one pair of diffraction spots at the spacing of 0.44 nm in Figure 2a, which can be indexed as 100 reflection. The absence or weakness of other reflections in the ED patterns indicates that these nylons crystallize into a triclinic structure similar to nylon 6 6 R phase,2-8 in which the molecular chains are tilted to the normal of the lamellar surface, and they cannot form the β phase of nylon,5-8 in which the molecular chains are normal to the lamellar surface. On the other hand, when the electron beam is parallel to the chain axis (c axis) by tilting the lamella 42° around the a* axis, another two pairs of diffraction spots with a spacing of 0.37 nm appear in Figure 2b, which can be indexed as 010 and 110 reflections,2-8 respectively. This kind of triclinic Rp crystalline structure has been reported for nylon 6 6,4,9,15 and many other even-even nylons.2-8 Figure 3 shows the WAXD patterns of these nylons, from which the diffraction data can be easily calculated. The d spacings from both ED and WAXD are all listed in Table 2, which shows that both of them agree with each other very well. Furthermore, the parameters of unit cell for these nylons can be determined by the following procedure.2-8 The a is set as 0.49 nm in accordance with the requirement that the hydrogen bonds should be linear within the chain-folded sheets. For the triclinic Rp phase the molecular chains shear by 13° progressively parallel to the chains, so β is 77°. The c value can be set at (0.125N - 0.02) nm, where N is the number of backbone bonds, which is consistent with an all-trans conformation of the molecular chains, while the 0.02 nm is subtracted due to the inclusion of one nitrogen atom in each backbone repeat unit. Then the b, R, and γ values can be calculated from the determined diffraction data of d100, d010, d110, and d001, and all of the unit cell parameters for nylons 4 22, 6 22, 8 22, 10 22, and 12 22 are listed in Table 3. Meanwhile, the angle between the electron beam and molecular chains can also be determined according to cos-1 θ ) d001/c, which shows the tilt angle of the nylon molecular chain from the lamellar normal, and they are listed in Table 4. It can be observed that nylons 4 22, 6 22, 8 22, 10 22, and 12 22 have similar unit cells to the R phase of nylon 6 64 except the c parameter because of the different lengths of the repeat unit. On the other hand, all the θ angles of these polymers in Table 4 are around 42°, which is in agreement with the tilting angle of lamellar crystals around a* axis when getting 010 and 110 reflections in ED patterns.
Morphology of Chain-Folded Lamellar Nylon Crystals
Crystal Growth & Design, Vol. 4, No. 2, 2004 385
Figure 2. Example of selected-area electron diffraction patterns for nylons 4 22, 6 22, 8 22, 10 22, and 12 22 (a) electron beam normal to the substrate, (b) titled 42° along the elongated axis of single crystal.
Figure 3. WAXD patterns of single-crystal mats for nylons 4 22, 6 22, 8 22, 10 22, and 12 22. Table 3. Unit Cell Parameters of Nylons 4 22, 6 22, 8 22, 10 22, 12 22, and 2 22 nylons
a (nm)
b (nm)
c (nm)
R (deg)
β (deg)
γ (deg)
nylon 4 22 nylon 6 22 nylon 8 22 nylon 10 22 nylon 12 22
0.49 0.49 0.49 0.49 0.49
0.53 0.55 0.54 0.54 0.54
3.48 3.73 3.98 4.23 4.48
48.6 46.4 47.4 47.1 48.1
77 77 77 77 77
63.1 63.3 63.1 63.3 63.0
nylon 2 22
0.49
0.52
3.23
59.2
77
59.4
Table 4. Angles between the Electron Beam and the Molecular Chains for Nylons 4 22, 6 22, 8 22, 10 22, 12 22, and 2 22 nylons θ angle (deg)
4 22
6 22
8 22
10 22
12 22
2 22
42.6
42.8
41.7
41.7
40.8
32.0
The WAXD plate photograph of sedimented crystal mats for nylon 10 22 is shown in Figure 4a. Moreover, the counterpart of the stretched film is also displayed in Figure 4b for the purpose of comparison. It can be seen that both of them have the same diffraction spacings, while there are the intense equatorial reflections indexed as 100 and 010/110 and the weak 00l
reflections in the meridian due to the orientation of the molecular chains during stretching in Figure 4b, which is also very similar to the R phase of nylon 6 6.1,15 However, due to low crystallinity during the crystallization process, the more perfect crystals cannot be obtained, which results in the disappearance of other diffraction spots in Figure 4b. Morphology and Crystalline Structure of Nylon 2 22. Figure 5 shows the morphology of lamellar crystals for nylon 2 22 which is lath-shaped and multilayered. Compared with other prepared nylons in this paper such as nylon 8 22, the lamellar crystals of nylon 2 22 are more regular, which displays more similarities to nylon 4 22. The long sides of the nylon 2 22 lamellar crystals are a little bent but have no serrated edges. Their dimension is approximately 1-2 µm in length and 200 nm in width. The ED patterns of nylon 2 22 reveal some similarities to other nylons in this series (Figure 2). Only one pair of diffraction spots indexed as 100 reflection can be observed in the ED pattern of nylon 2 22, which indicates that it also crystallizes into the triclinic Rp phase. However, its diffraction spacing is not 0.44 but 0.42 nm. On the other hand, if the lamella is tilted about 32° around the a* axis, another two pairs of diffraction signals with spacing of 0.39 nm appear, which can be indexed as 010 and 110 reflections, respectively. It can be seen that the diffraction patterns of nylon 2 22 also show some differences with nylon 6 64 and other prepared nylons, and it is more similar to those of the nylon 2 Y family such as nylon 2 6, 2 8, 2 10, 2 12,16 and 2 14.17 Their characteristic diffraction spacings are closer together than the corresponding values of 0.44 and 0.37 nm for nylon 6 6 and other synthesized nylons in this paper, which indicates that, although nylon 2 22 crystallizes into the triclinic structure, it adopts the crystalline structure between the triclinic Rp structure of nylon 6 6 and the pseudohexagonal system. Figure 6 shows the WAXD pattern of nylon 2 22 from which the diffraction spacings are calculated (see Table 2). Meanwhile, the data of ED are also listed in Table 2 which shows that both of them agree with each other very well. Because nylon 2 22 also crystallizes into the
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Figure 5. Transmission electron micrograph of the lamellar crystal of nylon 2 22.
Figure 6. WAXD pattern of single-crystal mats for nylon 2 22.
Conclusion
Figure 4. WAXD plate photographs of (a) sedimated mats and (b) stretched film for nylon 10 22.
triclinic Rp crystalline form, the same method is applicable to the calculation of its unit cell parameters. Consequently, the a, c, and β are predetermined according to the model of triclinic Rp, then other parameters are calculated by using the data of d100, d010, d110, and d001, all of which are listed in Table 3. Apparently, the unit cell parameters of nylon 2 22 are different from those of other prepared nylons, but very similar to those of the nylon 2 Y family.16,17 On the other hand, the θ angle (the angle between electron beam and the molecular chains) of nylon 2 22 in Table 4 is only 32.0°, which is much less than that of nylon 6 6 15 and other synthesized nylons in this paper but close to the value of nylon 2 12 16 and nylon 2 14.17
The morphology and crystalline structure of the chain-folded lamellar crystals for nylon 2 22, 4 22, 6 22, 8 22, 10 22, and 12 22 have been investigated by TEM and WAXD. The synthesized nylons except nylon 2 22 have a similar crystalline structure, and all of them crystallize into the triclinic Rp phase-like nylon 6 6. In addition, the unit cell parameters of these nylons are calculated, which also shows the similarity to the triclinic structure of nylon 6 6 Rp phase. On the other hand, the crystalline structure of nylon 2 22 has also been investigated by TEM and WAXD. The results show that, although nylon 2 22 crystallizes into the triclinic Rp system too, its characteristic signals of diffraction are not 0.44 and 0.37 nm but 0.42 and 0.39 nm. Accordingly, its crystalline structure is different from other prepared nylons in this paper and some conventional even-even nylons but very similar to the nylon 2 Y family. In addition, its calculated unit cell parameters come to the same results. References (1) Xenopoulos, E.; Clark, E. S. In Nylon Plastic Handbook; Kohan, M. J., Ed.; Hanser Garner Publications: Cincinnati, 1995; Chapter 5, pp 109-119.
Morphology of Chain-Folded Lamellar Nylon Crystals (2) Jones, N. A.; Atkins, E. D. T.; Hill, M. J.; Cooper, S. J.; Franco, L. Polymer 1997, 38, 2689-2699. (3) Atkins, E. D. T.; Hill, M. J.; Jones, N. A.; Cooper, S. J. J. Polym. Sci. Part B: Polym. Phys. 1998, 36, 2401-2412. (4) Jones, N. A.; Atkins, E. D. T.; Hill, M. J. J. Polym. Sci. Part B: Polym. Phys. 2000, 38, 1209-1221. (5) Jones, N. A.; Atkins, E. D. T.; Hill, M. J. Macromolecules 2000, 33, 2642-2650. (6) Atkins, E. D. T.; Hill, M.; Hong, S. K.; Keller, A.; Organ, S. Macromolecules 1992, 25, 917-924. (7) Hill, M. J.; Atkins, E. D. T. Macromolecules 1995, 28, 604609. (8) Jones, N. A.; Atkins, E. D. T.; Hill, M. J.; Cooper, S. J.; Franco, L. Macromolecules 1997, 30, 3569-3578. (9) Bunn, C. W.; Garner, E. V. Proc. R. Soc. (Lond) 1947, 189A, 3279-3284. (10) Ramesh, C. Macromolecules 1999, 32, 3721-3726.
Crystal Growth & Design, Vol. 4, No. 2, 2004 387 (11) Yan, D. Y.; Li, Y. J.; Zhu, X. Y. Macromol. Rapid Commun. 2000, 21, 1040-1043. (12) Li, Y. J.; Yan, D. Y.; Zhu, X. Y. Macromol. Rapid Commun. 2000, 21, 1282-1285. (13) Zhang, G. S.; Li, Y. J.; Yan, D. Y. Polym. Eng. Sci. 2003, 43, 470-478. (14) Li, W. H.; Yan, D. Y. Cryst. Growth Des. 2003, 3, 531-534. (15) Atkins, E. D. T.; Keller, A.; Sadler, D. M. J. Polym. Sci. Part 2 1972, 10, 863-875. (16) Jones, N. A.; Cooper, S. J.; Atkins, E. D. T.; Hill, M. J.; Franco, L. J. Polym. Sci. Part B: Polym. Phys. 1997, 35, 675-688. (17) Li, Y. J.; Zhang, G. S.; Yan, D. Y.; Zhou, E. L. J. Polym. Sci. Part B: Polym. Phys. 2002, 40, 1913-1918.
CG034177R
